I've rarely bothered with voiced mods, since I am that retard that reads the subtitles and skips the spoken dialogue (at least in TES), but from my experience the main problem is that their mics or audio balancing are usually shit, even within the same mod you get your ears crunched by someone recording on their phone while walking the dog with their friend managing to whisper while also talking at full volume. Also, after enough (too many) hours in Oblivion - any voice that isn't from the original cast just takes me out of the experience.
Truth be told - most of it probably would be forgiven if those shitlords would stop taking themselves so seriously. It's okay to be bad at voice acting, it's a mod, nobody is going to send a package filled with nothing but seventeen thousand printouts of goatsee because your 3 quest mod doesn't meet the AAA standard.